1. Field of the Invention
The present invention relates to a fuel cell system and more particularly to a fuel cell system using a proton exchange membrane as an electrolyte.
2. Description of the Prior Art
A proton exchange membrane fuel cell comprises a proton exchange membrane (PEM) between two electrodes, that is a cathode to which an oxidizing gas is supplied and an anode to which fuel gas is supplied. PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode, in the form of proton (H.sup.+). Each of the electrodes comprises a catalyst layer deposited on a porous base member through which the reactant gas is supplied. Mounted externally of each electrode is a separator or connector plate with grooves permitting the reactant gas to be introduced into the electrode at a constant flow rate. Excess gas which has not been consumed by the fuel cell reaction is exhausted to the open air through the grooved separator. The electricity generated by the energy conversion reaction at the anode is collected at the electrode porous base member and transported to the outside of the fuel cell system through the separator. In actual application, the system includes a plurality of fuel cells which are stacked in series with the separator being interposed between adjacent fuel cells.
Since the fuel cell generates heat in correspondence to the electric power generated, a fuel cell stack 2 usually includes cooling plates 803 between fuel cells 801, 801 at predetermined intervals, as shown in FIG. 9. Each cooling plate has a passage of a cooling medium such as air and water to prevent excessive overheat of fuel cells 801 in operation.
Proton is hydrated when being transferred through PEM electrolyte, so that PEM tends to be dehydrated as the fuel cell reaction proceeds. PEM must always be properly humidified to prevent decrease of ion-conductivity and energy conversion efficiency. In the conventional designs, hydrogen gas is humidified by suitable means which, in turn, humidifies the PEM when it is supplied to the anode.
Various attempts have also been proposed to humidify air to be supplied to the cathode. Since the cathode of the fuel cell in operation has been heated to 80.degree. C., for example, the air of a normal temperature should be preheated by a humidifier so that its saturated vapor becomes consistent with the ambient vapor condition of the cathode. Such a humidifier that is required to have water supplying function and air preheating function can not be simple in construction.
In Japanese patent un-examined publication No. 7-14599, there is provided a water injection nozzle to inject a necessary quantity of water into an air introducing pipe through which an air is supplied to the cathode of the PEM fuel cell. Since the nozzle is located upstream of a compressor, liquid water injected from the nozzle is evaporated when subjected to heat generated by the compressor. Thus, the cathode is humidified by vapor, not by liquid water.
In the fuel cell system of Japanese patent un-examined publication No. 9-266004, a discharge gas from the anode containing hydrogen gas which has not been consumed during the anodic reaction is introduced into the cathode where the unconsumed hydrogen gas in the discharge gas is combusted with oxygen to generate water, which well humidifies PEM electrolyte. In this system, there is no need to install a humidifier for humidifying air to be supplied to the cathode.
During operation of the fuel cell system, a proton produced at the anode is moved to the cathode where it reacts with oxygen in the air or any other oxidizing gas supplied thereto to produce water. Accordingly, in accordance with the conventional recognition in the art, there is a greater need to humidify hydrogen gas to be supplied to the anode, than at the cathode where water can at least partially be self-sustaining.
As a result of the inventors' repeated tests and investigation, however, it has been found that water produced at the cathode permeates through PEM electrolyte toward the anode, which makes it unnecessary to humidify hydrogen gas to be supplied to the anode. On the other hand, a water content of the PEM electrolyte at the cathode side tends to decrease by contacting the air flow the cathode. Such finding is contradictory to the conventional knowledge and has been first recognized by the present inventors.